A typical metal casting process is described in U.S. Pat. No. 3,111,732. In that process, liquid metal is poured through a spout (or “underpour outlet”) into a mould, where the metal freezes to form a billet or slab. The flow of metal through the spout is controlled by a control pin (or “flow regulator”) that is located within the spout. The control pin may be raised to increase the rate of flow of metal through the spout, or lowered to decrease or interrupt the flow of metal.
Control pins are generally made of a refractory material, which is able to withstand the high temperature of the molten metal. The material must also be hard so as to resist wear on the end of the rod, where it presses against the seat in the spout. One of the most commonly used materials is dense fused silica (DFS). This material is quite tough and has good thermal shock characteristics, but silica is wetted and attacked by liquid aluminium and control pins made of this material therefore have to be provided with a non-stick protective coating, for example of boron nitride. This coating has to be reapplied frequently (for example every one or two pouring operations) and such pins therefore have a high maintenance requirement.
Another disadvantage with control pins made of DFS is that they tend to have a high heat capacity and have to be pre-heated prior to commencement of the metal pouring operation, to bring them up to or close to the temperature of the molten metal. This adds considerably to the complexity of the pouring operation and gives rise to the risk of a serious accident when transferring the hot control pin from the pre-heating oven to the spout. If the control pin is not pre-heated, the molten metal can solidify upon contact with the control pin, thus blocking the spout.
Our earlier patent application EP1525936 describes a control pin for controlling the flow of liquid metal in a casting process, which includes an elongate body member and a wear-resistant tip at one end of the elongate body member, the body member being made at least partially of a laminated composite ceramic material that includes multiple layers of a reinforcing fabric embedded within a cast ceramic matrix. The control pin resolves most of the disadvantages set out above and has proved to be extremely durable, having a designed service lifetime of approximately 40 drops, as compared to a lifetime of typically just 15 drops for a control pin made of DFS.
This very long lifetime has, however, led some users to ignore the designed service lifetime and use the control pin for much longer, for example for 60 or more drops. Repeated exposure to the high temperature of the liquid aluminium can cause carbonisation and degradation of the reinforcing fabric, eventually causing the control pin to break. The broken part of the control pin can then block the pouring spout, causing serious operational difficulties. The present invention provides a control pin that mitigates at least some of the aforesaid disadvantages.